Threshold voltage sensor and a chip using the threshold voltage sensor

1 . A threshold voltage sensor, comprising:

a first metal-oxide-semiconductor field-effect transistor (MOSFET), a second MOSFET, and an operational amplifier, wherein the operational amplifier has an output terminal coupled to a gate terminal of the first MOSFET and a gate terminal of the second MOSFET, a first input terminal receiving a first constant voltage, and a second input terminal coupled to a drain terminal of the first MOSFET;

an active device, coupled between the drain terminal of the first MOSFET and a ground terminal to determine a constant current that is mirrored from the first MOSFET to the second MOSFET, wherein the active device comprises a third MOSFET coupled between the drain terminal of the first MOSFET and the ground terminal, and the third MOSFET has a gate terminal biased by a second constant voltage;

a diode-connected MOSFET, coupled between a drain terminal of the second MOSFET and the ground terminal; and

a fourth MOSFET, coupled in series with the diode-connected MOSFET, and having a gate terminal biased by a third constant voltage;

wherein the constant current flows through the diode-connected MOSFET to generate an output voltage that contains information about a threshold voltage of the diode-connected MOSFET.

2 . The threshold voltage sensor as claimed in claim 1 , wherein:

the active device further comprises a first series of MOSFETs coupled between a source terminal of the third MOSFET and the ground terminal.

3 . The threshold voltage sensor as claimed in claim 2 , further comprising:

a second series of MOSFETs coupled between a source terminal of the fourth MOSFET and the ground terminal;

wherein the drain terminal of the second MOSFET is coupled to a drain terminal of the fourth MOSFET through the diode-connected MOSFET.

4 . The threshold voltage sensor as claimed in claim 3 , wherein:

the third MOSFET and the diode-connected MOSFET have the same length-to-width ratio;

the third MOSFET and the fourth MOSFET are the same size; and

the size and number of the first series of MOSFETs are the same as the size and number of the second series of MOSFETs.

5 . The threshold voltage sensor as claimed in claim 4 , wherein:

gate terminals of the first series of MOSFETs are biased by the second constant voltage; and

gate terminals of the second series of MOSFETs are biased by the third constant voltage.

6 . The threshold voltage sensor as claimed in claim 5 , wherein:

the first constant voltage equals the second constant voltage which equals the third constant voltage; and

the third MOSFET and the fourth MOSFET are operated in a saturation region.

7 . The threshold voltage sensor as claimed in claim 5 , wherein:

the second constant voltage equals the third constant voltage, and is greater than the first constant voltage; and

the third MOSFET and the fourth MOSFET are operated in a linear region.

8 . The threshold voltage sensor as claimed in claim 1 , further comprising:

a fifth MOSFET, coupled in series with the third MOSFET, and having a gate terminal biased by a fourth constant voltage.

9 . The threshold voltage sensor as claimed in claim 8 , wherein:

the first constant voltage equals the second constant voltage which equals the third constant voltage; and

the third MOSFET, the fourth MOSFET, and the diode-connected MOSFET MD are operated in a saturation region, and the fifth MOSFET is operated in a linear region.

10 . The threshold voltage sensor as claimed in claim 8 , wherein:

the second constant voltage equals the third constant voltage, and is greater than the first constant voltage; and

the third MOSFET, the fourth MOSFET, and the diode-connected MOSFET MD are operated in a saturation region, and the fifth MOSFET is operated in a linear region.

11 . The threshold voltage sensor as claimed in claim 8 , wherein:

the fifth MOSFET is an n-channel MOSFET (NMOS);

the third MOSFET and fourth MOSFET are NMOSs; and

the diode connected MOSFET is implemented by an NMOS and is coupled between a source terminal of the fourth MOSFET and the ground terminal.

12 . The threshold voltage sensor as claimed in claim 11 , wherein:

the first bias voltage, second bias voltage, third bias voltage, and fourth bias voltage make the third MOSFET, the fourth MOSFET, and the diode-connected MOSFET operating in a saturation region, and the fifth MOSFET operating in a linear region.

13 . The threshold voltage sensor as claimed in claim 8 , wherein:

the fifth MOSFET is a p-channel MOSFET (PMOS);

the third MOSFET and fourth MOSFET are NMOSs; and

the diode connected MOSFET is implemented by a PMOS and is coupled between a source terminal of the fourth MOSFET and the ground terminal.

14 . The threshold voltage sensor as claimed in claim 13 , wherein:

the first bias voltage, second bias voltage, third bias voltage, and fourth bias voltage make the third MOSFET, the fourth MOSFET, and the diode-connected MOSFET operating in a saturation region, and the fifth MOSFET operating in a linear region.

15 . The threshold voltage sensor as claimed in claim 1 , wherein:

the third MOSFET is an NMOS; and

the diode connected MOSFET is implemented by an NMOS.

16 . The threshold voltage sensor as claimed in claim 1 , wherein:

the third MOSFET is a PMOS;

the diode connected MOSFET is implemented by a PMOS.

17 . A chip, comprising:

an analog block;

the threshold voltage sensor as claimed in claim 1 , placed within a surrounding area of the analog block;

an analog-to-digital converter, converting the output voltage generated by the threshold voltage sensor to a digital value; and

a digital signal processor, performing a calibration process for the analog block based on the digital value.